Recently, energy saving orientation for each industrial product is getting stronger in terms of environmental problems, a crude oil price hike and others. A field of working vehicles including construction vehicles that have principally used a hydraulic driving system using a diesel engine for a power source also has the trend, and cases of the enhancement of efficiency by motorization and energy saving increase.
For example, when a driving section of the above-mentioned working vehicle is motorized, multiple energy saving effects such as efficient driving of an engine (in the case of a hybrid type), the enhancement of power transmission efficiency and the recovery of regenerated power in addition to the reduction of exhaust gas can be expected. In such a field of working vehicles, the motorization of a fork lift truck makes most progress, a so-called battery fork lift truck that drives a motor using electric power of a battery is promptly practicalized prior to other vehicles, and motorization is rather popularized among working vehicles with a compact fork lift truck in the center.
Uninterruptedly, recently, in a field of a hydraulic excavator, an engine type fork lift truck and others, a “hybrid vehicle” in which a diesel engine and an electric motor are combined starts to be productized. Further, a wheel loader can be given for a vehicle for which relatively great fuel efficiency reduction effect can be expected as effect in the case of coming hybridization out of working vehicles in which correspondence to environment by motorization and energy saving advance as described above.
FIG. 12 shows one example of a configuration of a wheel loader 100 heretofore known.
As shown in FIG. 12, the wheel loader 100 according to the related art is provided with an engine 101, a torque converter 102 that transmits power of the engine 101, a transmission (T/M) 103 that shifts the power transmitted from the torque converter 102, a propeller shaft 104 connected to the transmission 103, a hydraulic pump 105 driven by the engine 101 and a hydraulic working mechanism 106 operated by pressure oil discharged from the hydraulic pump 105.
The wheel loader 100 can excavate and carry earth, sand and others with a bucket of the hydraulic working mechanism 106 on the front side for example, transmitting the power of the engine 101 to an axle by the torque converter 102 and the transmission 103 via the propeller shaft 104 and traveling. For a typical work mode of the wheel loader 100 configured as described above, V-shaped excavation work can be given.
FIG. 13 illustrates the V-shaped excavation work.
As shown in FIG. 13, first, the wheel loader 100 advances toward a target to be excavated 110 such as a pile of gravel, moves backward after gravel and others are loaded onto the bucket of the hydraulic working mechanism 106 by thrusting the bucket into the target to be excavated 110, and returns to a home position. Afterward, when an operator manipulates a steering wheel and an operating lever, the wheel loader 100 advances toward a haulage vehicle 111 such as a dump truck, lifting the bucket of the hydraulic working mechanism 106. The wheel loader 100 moves backward again after the haul is loaded onto the haulage vehicle 111, that is, after the haul is released from the bucket and returns to the home position. As described above, the wheel loader 100 repeats the V-shaped excavation work, drawing a V-shaped locus.
In the above-mentioned V-shaped excavation work, the wheel loader 100 distributes the power of the engine 101 to a traveling section and the hydraulic working mechanism 106 on the front side, travels, and performs the V-shaped excavation work. This V-shaped excavation work requires great power because in the traveling section, great tractive force is exercised, the bucket is thrusted into the target to be excavated 110 and afterward, a pile of haul is scooped with the bucket of the hydraulic working mechanism 106 on the front side. Besides, as the wheel loader 100 simultaneously operates in the traveling section and the hydraulic working mechanism 106 when the wheel loader is accelerated toward the haulage vehicle 111, lifting the bucket in a state in which the haul is loaded, the wheel loader requires great power again.
Accordingly, when the wheel loader 100 performs the V-shaped excavation work which is a basic operational pattern, the increase and decrease of great power occur. When power is supplied by the engine 101 according to a load condition of the V-shaped excavation work described above, the vehicle is not necessarily operated at a satisfactory fuel efficiency point in a fuel efficiency characteristic of the engine 101 in a case that the vehicle is not driven at suitable speed of the engine 101 and consequently, the wheel loader may be “a vehicle of poor fuel efficiency”. In the wheel loader 100 according to the related art provided with the above-mentioned torque converter 102 shown in FIG. 12, as the speed corresponding to a load of the engine 101 is basically determined by characteristics of the torque converter 102 and the hydraulic pump 105, the speed of the engine 101 cannot be arbitrarily set.
In the meantime, as a hybrid wheel loader has a configuration that an engine and a propeller shaft are not mechanically connected for example, engine speed can be arbitrarily varied according to an operation of the vehicle. As a secondary battery such as a lithium battery and an electrical storage device such as an electric double layer capacitor are mounted in the hybrid wheel loader, it is conceivable that the operation of the engine can be controlled by using electric power of the electrical storage device for a supply source of power required for the operation of the vehicle so that fuel efficiency of the engine is as satisfactory as possible.
For one of the related arts of this type hybrid working vehicle, there is proposed a hybrid working machine mounting a hybrid control system equipped with a first driving machine provided with only a powering function, a second driving machine provided with powering and regenerating functions, a power unit that charges regenerative energy from the second driving machine and can discharge electrical energy into the second driving machine and a controller that instructs the first driving machine to supply power up to maximum output power of the first driving machine to a load in powering, instructs the power unit to supply quantity of the load exceeding the maximum output power of the first driving machine via the second driving machine and instructs to restore regenerative energy from the load to the power unit via the second driving machine in regeneration (for example, refer to a patent literature 1).